Remove concept of 'completion' from the projection cache

Fixes rust-lang#88910

When we initially store a `NormalizedTy` in the projection cache,
we discard all obligations that we can (while ensuring that we
don't cause any issues with incremental compilation).

Marking a projection cache entry as 'completed' discards all
obligations associated with it. This can only cause problems,
since any obligations stored in the cache are there for a reason
(e.g. they evaluate to `EvaluatedToOkModuloRegions`).

This commit removes `complete` and `complete_normalized` entirely.

compiler/rustc_infer/src/traits/project.rs

@@ -153,47 +153,6 @@ impl<'tcx> ProjectionCache<'_, 'tcx> {
         assert!(!fresh_key, "never started projecting `{:?}`", key);
     }
 
-    /// Mark the relevant projection cache key as having its derived obligations
-    /// complete, so they won't have to be re-computed (this is OK to do in a
-    /// snapshot - if the snapshot is rolled back, the obligations will be
-    /// marked as incomplete again).
-    pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
-        let mut map = self.map();
-        let ty = match map.get(&key) {
-            Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
-                debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}", key, ty);
-                ty.value
-            }
-            ref value => {
-                // Type inference could "strand behind" old cache entries. Leave
-                // them alone for now.
-                debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}", key, value);
-                return;
-            }
-        };
-
-        map.insert(
-            key,
-            ProjectionCacheEntry::NormalizedTy(Normalized { value: ty, obligations: vec![] }),
-        );
-    }
-
-    /// A specialized version of `complete` for when the key's value is known
-    /// to be a NormalizedTy.
-    pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
-        // We want to insert `ty` with no obligations. If the existing value
-        // already has no obligations (as is common) we don't insert anything.
-        if !ty.obligations.is_empty() {
-            self.map().insert(
-                key,
-                ProjectionCacheEntry::NormalizedTy(Normalized {
-                    value: ty.value,
-                    obligations: vec![],
-                }),
-            );
-        }
-    }
-
     /// Indicates that trying to normalize `key` resulted in
     /// ambiguity. No point in trying it again then until we gain more
     /// type information (in which case, the "fully resolved" key will

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -14,18 +14,17 @@ use super::util;
 use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
 use super::wf;
 use super::DerivedObligationCause;
+use super::Normalized;
 use super::Obligation;
 use super::ObligationCauseCode;
 use super::Selection;
 use super::SelectionResult;
 use super::TraitQueryMode;
-use super::{Normalized, ProjectionCacheKey};
 use super::{ObligationCause, PredicateObligation, TraitObligation};
 use super::{Overflow, SelectionError, Unimplemented};
 
 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
 use crate::traits::error_reporting::InferCtxtExt;
-use crate::traits::project::ProjectionCacheKeyExt;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_data_structures::sync::Lrc;
@@ -574,14 +573,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     match project::poly_project_and_unify_type(self, &project_obligation) {
                         Ok(Ok(Some(mut subobligations))) => {
                             self.add_depth(subobligations.iter_mut(), obligation.recursion_depth);
-                            let result = self
-                                .evaluate_predicates_recursively(previous_stack, subobligations);
-                            if let Some(key) =
-                                ProjectionCacheKey::from_poly_projection_predicate(self, data)
-                            {
-                                self.infcx.inner.borrow_mut().projection_cache().complete(key);
-                            }
-                            result
+                            self.evaluate_predicates_recursively(previous_stack, subobligations)
                         }
                         Ok(Ok(None)) => Ok(EvaluatedToAmbig),
                         Ok(Err(project::InProgress)) => Ok(EvaluatedToRecur),